In the paper industry, cardboard sheets are manufactured starting from a continuous web like cardboard material, which is slit longitudinally and divided into strips. Each strip is further divided transversely to generate a plurality of sheets of desired length. Sheets thus obtained are delivered to a so-called stacker or stacking apparatus, which forms stacks or bundles of sheets. The stacks are subsequently delivered to the final user, for example for the manufacturing of cardboard boxes or the like. Small bundles can be combined into larger stacks before shipping.
Fast advancing sheets must be carefully piled up to form stacks of regular shape. Known stacking apparatuses usually comprise a sheet conveyor arrangement which receives a substantially continuous flow of sheets which are shingled and delivered onto a stacking surface in a stacking bay.
In some cases, each stack is formed by staggered bundles, each bundle containing a predetermined number of sheets. TW-M423688U, US2014/0353119 and US2009/0169351 disclose sheet stackers configured and controlled for forming stacks of mutually staggered bundles of cardboard sheets. In order to mutually stagger neighboring bundles of the stack, said stack is formed on a horizontally movable stacker platform. The reciprocating staggering motion is in a direction substantially parallel to the feed direction of the cardboard sheets. The stacker platform comprises a conveyor belt, forming a stacking surface. The conveyor belt has a horizontal conveying motion, orthogonal to the reciprocating staggering motion of the stacker platform. The conveyor belt is used to evacuate the formed stack from the stacking bay according to an evacuation direction which is substantially orthogonal to the direction of arrival of the cardboard sheets in the stacking bay. Each bundle of a stack is formed against a single stop plate or a dual stop plate, which are arranged in two positions which are staggered along the direction of arrival of the cardboard sheets. Staggering of neighboring bundles is obtained by means of a reciprocating motion of the stacker platform in a horizontal direction. Moving the entire stacker platform is difficult and requires strong actuators and a particularly sturdy structure.
CN204057396U and CN203255778U disclose further embodiments of stackers designed and configured for producing stacks of sheets, each formed by a plurality of staggered bundles. Staggering is obtained by using two mutually spaced apart stop plates. The distance between the stop plates is equal to the staggering of neighboring bundles. In addition to moving the stop plates, the sheet discharge end of the sheet conveyor must also be reciprocatingly moved back and forth in a direction parallel to the feed direction, to achieve correct staggering of adjacent bundles.
In other known sheet stackers, smooth stacks are formed, as disclosed e.g. in U.S. Pat. No. 4,273,325.
U.S. Pat. No. 5,829,951 discloses an up-stacker, i.e. a sheet stacker wherein the stacks are formed on a stationary stacker platform, and wherein a sheet conveyor arrangement is provided having a downstream sheet discharge end, wherefrom the sheets are discharged onto the stack being formed, moves gradually upwards as the stack grows vertically. This known stacker is suitable for the formation of small stacks or bundles of sheets.
One of the critical aspects of sheet stackers is the transient phase of removing the formed stack from the stacking bay. Removing the stack requires a gap to be formed in the otherwise continuous flow of sheets delivered by the sheet conveyor arrangement to the stacking bay. The longer the time required for removing a just formed stack of sheets from the stacking bay, the larger the gap required in the sheet flow. This transient phase slows down the operation of the sheet stacker and thus adversely affects the mean production rate thereof. Also, forming a large gap in the sheet flow can be difficult.
A need therefore exists, for reducing the time of such transient phases.